Catalysts play key roles in most of the important chemical reactions, including syntheses of organic compounds, reforming production of hydrogen from methanol, natural gas and gasoline, catalytic conversion of carbon monoxide, nitrogen oxides and unburned fuels, etc. In particular, platinum and platinum alloys are indispensable catalysts for direct methanol and proton exchange membrane fuel cells with respect to electro-reduction of oxygen and electro-oxidation of fuel. So far, most of the catalysts are prepared using suitable supports, such as carbon blacks, ceramics and silica gels. The so-called nanocatalysts are those having particles prepared in nano-sizes, generally a few nanometers.
In preparing catalysts for DMFC and PEMFC applications, high surface area carbon supports, such as Cabot Vulcan XC-72 and Chevron Shawinigan, are commonly used. Typical examples of state-of-the-art for making carbon black-supported platinum and platinum alloy catalysts are given by Cooper et al. U.S. Pat. No. 5,316,990, Brand et al. U.S. Pat. No. 5,489,563, Gunner et al. U.S. Pat. No. 5,939,220 and Auer et al. U.S. Pat. No. 6,007,934. In general, catalyst metal in ions or colloidal sols are adsorbed to carbon support surfaces and reduced by chemical reactions. High temperature reduction using hydrogen gas is also commonly used. However, such prepared nanocatalysts suffer from drawbacks of re-construction, re-crystallization and sintering when in use to form larger particles and substantially lose their catalytic surfaces. These give rise to significantly lower catalytic efficiencies and shorter service lives. Novel approaches are needed to prepare and stabilize catalysts under all application conditions.
Due to the advancement of nanotechnologies, nanostructures can be readily fabricated. In one example, Billliet et al., U.S. Pat. No. 6,709,622 illustrates a method to fabricate nanostructures containing isotropically distributed, interconnected pores having cross-sectional diameters in the nanometer and Angstrom ranges. Briefly, inorganic nanoparticles are used as a precursor and mixed with organic polymer to form a homogeneous thermoplastics compound. The organic polymer is then removed by sintering at a high temperature and a nanostructure is formed. In another example shown by Jin, U.S. Pat. No. 6,858,521, a spaced-apart array of nanostructures is fabricated by providing a show mask having a plurality of spaced apart, relatively large apertures, reducing the size of the aperture to nanoscale dimensions, and depositing a material through the mask to form a plurality of spaced-apart nanostructures. However, in general these nanostructures are suitable for used as catalyst supports only rather than as catalysts. It would be very beneficial if a catalyst itself can be fabricated into a nanostructure with excellent resistances to recombination, recrystallization and sintering wile providing high surface area and high catalytic efficiency.
In fact, using some nanomaterials, e.g., polystyrene nanospheres and silicon dioxide nanospheres, as structure-directing agents do provide a new approach to achieve this goal. These nanospheres readily form closely packed, layered structures of one to several layers on a smooth support surface when applied at diluted conditions. With these unique characteristics of such nanomaterials, this invention is able to prepare novel catalysts with nanonetwork structures on suitable supports that are totally different from those of the conventional nano catalysts. The advantages are to overcome the drawbacks encountered by conventional nanocatalysts. In addition, the platinum and platinum alloys catalysts prepared by the invented methods will significantly improve the performances of DMFCs and PEMFCs due to robust structure, resistant to recombination, and high catalytic surfaces.